Most oil and gas wells are drilled using drill pipe. At selected depths and at the total depth, the operator removes the drill pipe and drill bit, then lowers a string of casing into the wellbore and cements it in place. If the operator needs to replace the drill bit, he trips the drill bit out of the wellbore with the drill pipe, then runs the drill bit back into the wellbore with the drill pipe.
Another approach, referred to as casing drilling, involves the use of the casing as the drill string rather than drill pipe. The drill bit may be secured to the lower end of the casing and cemented in place along with the casing after reaching the desired depth. Alternately, the operator may use a retrievable drill bit assembly. The drilling is accomplished by rotating the casing with a top drive of the drilling rig. Drilling may also be accomplished using a mud motor, which rotates the drill bit relative to the casing.
In current techniques, retrievable drill bits are connected to a drill lock assembly, which is run through the casing to a profile sub at the lower end of casing. The drill lock assembly has a set of stop dogs that are biased outward and land on a shoulder provided in the profile sub. The drill lock assembly also has torque keys that are biased outward for engaging longitudinal slots provided in the profile sub. In addition, the drill lock assembly has an axial lock mechanism that is moved outward into engagement with a profile in the profile sub to prevent upward movement of the drill lock assembly in the profile sub. The drill lock assembly may be run by wireline, drill pipe, or it may be pumped into and out of the casing. If the drill lock assembly is to be run by wireline, a wireline running tool is used to lower the drill lock assembly into the profile sub, set it, then release from it to be retrieved back to the surface.
To retrieve the drill lock assembly by wireline, the operator attaches a retrieval tool to the wireline, lowers it into engagement with the drill lock assembly, releases the drill lock assembly from the profile sub, and retrieves it to the surface. Similarly, if drill pipe is utilized, a retrieval tool is employed for retrieving.
Generally, the drill lock assembly and running tools are complex, having many parts that must operate correctly. The environment in which the drill lock assembly is located is severe, having drilling mud and cuttings that may accumulate on top of the drill lock assembly. Further, lost circulation material is often added to mud used in casing drilling operations and it has the ability to hamper or foul operation of mechanical tools. Casing drilling is often used in wells with severe lost circulation problems. Provisions must be made for retrieving the drill lock assembly in the event that it is stuck and can not be retrieved without over pulling on the wireline.